There are industrial processes where electric induction heating of a continuous strip must be accomplished in a gas-tight tunnel furnace. For example as shown in longitudinal cross section in FIG. 1, strip 90 passes through electric induction gas-sealed tunnel furnace 110. Furnace enclosure 112 is made sufficiently gas-tight around the tunnel 114 through which strip 90 passes. Electric induction coil 116 (or coils) can be placed outside of enclosure 112 if the enclosure is sufficiently transparent to the magnetic flux field that is generated by alternating current flowing through coil 116 and allows the field to penetrate inside of the enclosure so that the field can magnetically couple with the strip in the tunnel. Thermal insulation 118 can be utilized, for example, between the interior of the tunnel and enclosure 112. The flux field heats the strip by electromagnetically coupling with the strip to induce eddy currents in the strip. The strip is heated to perform an industrial process, for example, if the strip is coated with a liquid composition before entry into the tunnel, inductive heating of the strip will cause the liquid composition to bond (or cure) to the strip by evaporation of solvents in the liquid composition.
In some industrial processes the inductive heating in the furnace must be accomplished in a process gas environment that could be problematic if the tunnel gas is released into the open air (atmosphere) around the outside of the furnace for reasons such as pollution, explosive or combustive reaction with air, high cost of the process gas, or strict low tolerance to deviations in the composition of the process gas. For example the process gas in the tunnel for decarburization of steel comprises a high concentration hydrogen gas. Although enclosure 112 may be called a “gas-tight” enclosure, the enclosure is subject to leakage since, practically, the enclosure can not be constructed as a single continuous enclosure without the cost being prohibitive. Therefore there are, for example, joints between materials making up the enclosure that may be sufficiently gas-tight during initial fabrication of the enclosure, but may leak after the furnace is put into operation, for example, as a result of repeated heating and cooling of the materials around the joint. Also the enclosure composition and thermal insulation themselves may be gas permeable and serve as passages for gas leaks from the tunnel. One method of handling tunnel gas leaks is to allow the leaking tunnel gas to escape into a well ventilated atmosphere. For example forced ventilation box 180 can be placed around the exterior of furnace 110. Top openings 180a in the ventilation box provide a directed release of gas from the ventilation box when fan 182 forces surrounding external air through the ventilation box. However such method lacks a precise means of insuring that dangerous concentrations of process gas do not build up in the atmosphere exterior to the furnace.
It is one object of the present invention to provide an electric induction gas-sealed tunnel furnace that will assist in preventing the release of a process gas from an electric induction gas-sealed tunnel furnace.